Heretofore, gate valves utilizing a reciprocating gate member have included a cast body member having a through-bore and a valve seat machined therein surrounding one of the ports in the through-bore and a bonnet attached to the body member which provides a chamber open to the through-bore and which contains the rising or non-rising valve stem for the gate member and space for the gate member when the valve is open. One such prior type of gate valve structure is that shown in the Common Assignees prior U.S. Pat. No. 3,662,778, issued May 16, 1972 to Leopold, Jr. et al. This type of gate valve included a gate member which was reciprocated from an open position where the gate member was out of the through-bore and the through-bore was completely unobstructed to thereby reduce turbulence and currents within the valve structure and provide for smoother flow with less pressure drop, to a position across the through-bore so as to close the valve. While the gate valve was of the resilient seated type in that the gate member was provided with a resilient covering, the valve seat was of a rather complicated shape and required an expensive operation to machine the same and, in fact, involved certain manufacturing difficulties since machining was accomplished either through the opening in the upper part of the casing or through the ports of the through-bore.
More recently, gate valve structures have been developed such as those disclosed in the Common Assignees prior U.S. Pat. No. 3,963,214, issued June 15, 1976 to Hackman et al. This type of gate valve included a valve seat surrounding one of the ports in the through-bore and lying generally in a plane converging at an acute angle to a plane normal to the axis of the through-bore. The patent included a gate member having an annular seal member thereon, the seal member having a bulbous portion surrounding its entire periphery for engaging the valve seat, the valve seat having a planar surface facing the other port and a bottom surface merging into and forming part of the through-bore. However, it has been found that difficulties are encountered with this type of seat arrangement as an edge is created at the intersect of the seat with the area of the casing surrounding the through-bore and as a result of the uniformed shaped annular seal member, the same dragged across this edge resulting in tearing or cutting of the same during closing and opening of the valve and particularly in closing when a wedging action is created on the upper portion of the annular seal member as the lower portion is placed under compression with the bottom of the through-bore. The valve casing in this prior art type of gate valve was formed of two body members which were split on a plane normal to the axis of the through-bore, the two body members being secured to each other to define the chamber and the through-bore and one body member contained the valve seat. It was thought that by providing the valve casing split in this manner it would be easier to machine the valve seat in one of the body members. However, such machining of this type of seat can be accomplished in the conventional type of gate valve wherein the valve casing includes a body member defining the seat and the through-bore and a bonnet for defining the chamber for the valve operating mechanism and for the gate member when the valve is in the open position.
The gate member of the gate valve structure disclosed in the aforementioned U.S. Pat. No. 3,963,214 provided an arcuate shoulder which axially extended from the planar seating surface for the annular seal member, the arcuate shoulder being arranged to enclose the outer periphery of the annular seal member on its upper portion. With such an arrangement and since there was wedging action created during closing on the upper portion of the seal member, this wedging action was somewhat like a shearing action and consequently, there was a flow of the resilient seal member. However, this flow was such that it could not take place within the shoulder area and since there was no place for the material of the seal member to go, it was cut on the sharp circumferential ends of the arcuate shoulder. This results in the valve having a damaged sealing surface on the seal member after a number of openings and closings, thus, requiring the seal member to be changed. This damage to the seal member was in addition to the damage caused by the seal member dragging across the edge created during the machining of the valve seat.